drm/mediatek: Fix Kconfig warning

WARNING: unmet direct dependencies detected for MTK_MMSYS
  Depends on [n]: (ARCH_MEDIATEK [=y] || COMPILE_TEST [=n]) && COMMON_CLK_MT8173_MMSYS [=n]
  Selected by [y]:
  - DRM_MEDIATEK [=y] && HAS_IOMEM [=y] && DRM [=y] && (ARCH_MEDIATEK [=y] || ARM && COMPILE_TEST [=n]) && COMMON_CLK [=y] && HAVE_ARM_SMCCC [=y] && OF [=y]

Make DRM_MEDIATEK depend on MTK_MMSYS to fix this.

Fixes: 2c758e30

 ("soc / drm: mediatek: Move routing control to mmsys device")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>